Unit test the tricky parts

How to do it

Keep small, fast tests next to the code for anything with branches or calculations, such as parsing, prices and dates. Run them before every build. For more depth, see the testing playbook.

Example

// date.test.js, run with: node --test
import test from 'node:test';
import assert from 'node:assert/strict';
import { formatDate } from './date.js';

test('formats a date for people', () => {
  assert.equal(formatDate('2026-10-20'), '20 October 2026');
});

How to check

The tests run in seconds, in the full check before every deploy. Break the logic on purpose, and a test fails.